Hibernate from a non activate swapfile from btrfs

* Important! * This didn't work! It is not secure! When wiping happens, the file gets moved to another place by btrfs, then, this doesn't work for two reasons: 1) Wiping happens in other parts of the disk, and the offset changes and has to be updated in all configurations files after wiping everytime.

* This was just a stupid experiment.

Create swapfile

Create a separate swap volume (not necessary)

sudo btrfs subvolume create /mnt/btrfsbase/swap

* Add it to /etc/fstab

UUID=8f9ede3a-912f-40d6-9335-90ed3186e3e4   		      	    /swap	      	     btrfs defaults,noautodefrag,nodatacow,nodiscard,noatime,subvol=swap	0 0

Be sure there is no discard nor autodefrag options activated for this subvolume

Create a swapfile

cd /swap
sudo truncate -s 0 swapfile
sudo chattr +C swapfile
btrfs property set swapfile compression none
sudo chmod 600 swapfile
sudo dd if=/dev/zero of=swapfile bs=1M count=24576 #reserve 24GB
sudo mkswap /swap/swapfile

Find out resume device and offset

Find out resume device:

sudo findmnt -no UUID -T /swap/swapfile

swap_offset command doesn't work correctly with btrfs. Download this https://github.com/osandov/osandov-linux/blob/master/scripts/btrfs_map_physical.c and compile it:

gcc -O2 -o btrfs_map_physical btrfs_map_physical.c

Calculate offset
Write down the last number from the output of the following command:

sudo btrfs_map_physical /swap/swapfile | head -n 2

Result:

FILE OFFSET	FILE SIZE	EXTENT OFFSET	EXTENT TYPE	LOGICAL SIZE	LOGICAL OFFSET	PHYSICAL SIZE	DEVID	PHYSICAL OFFSET
0	134217728	0	regular	134217728	4594864128	134217728	1	4594864128

Write down the output of the following command:

sudo getconf PAGESIZE

Result: 4096

Calculate offset: Divide the first number under the PAGESIZE: offset = PHYSICAL_OFFSET/PAGESIZE

4594864128/4096=1121793

Configure system for hibernation

Create /etc/initramfs-tools/conf.d/resume and put the resume and offset data on it:

RESUME=UUID=8f9ede3a-912f-40d6-9335-90ed3186e3e4 resume_offset=1121793

Modify /etc/default/grub with the corresponding resume and offset values:

GRUB_CMDLINE_LINUX_DEFAULT="resume=UUID=8f9ede3a-912f-40d6-9335-90ed3186e3e4 resume_offset=1121793 quiet btusb.enable_autosuspend=n"

Activate the swap file

sudo swapon /swap/swapfile

Install uswsusp package
Edit /etc/uswsusp.conf:

resume device = /dev/disk/by-uuid/8f9ede3a-912f-40d6-9335-90ed3186e3e4
early writeout = y
image size = 0
RSA key file = /etc/uswsusp.key
shutdown method = platform
resume offset = 1121793

Activate a systemd service:

sudo systemctl edit systemd-hibernate.service

With:

[Service]
ExecStart=
ExecStartPre=-/bin/run-parts -v -a pre /lib/systemd/system-sleep
ExecStart=/usr/sbin/s2disk
ExecStartPost=-/bin/run-parts -v --reverse -a post /lib/systemd/system-sleep

Update initramfs and grub

sudo update-grub
sudo update-initramfs -k all -u
sudo update-grub

Test the hibernation:

sudo s2disk

I have a system with encrypted swap, but I don't want to have encryption on the hibernation swap file. My idea is to use hibernation only when strictly necessary, because I don't want to need to put a password during booting. Then, this swap file should actually be always deactivated except before hibernation. When resuming, this swap space should be deactivated and then securely wiped! The script to do this would be this: my_server_hibernate.sh:

#!/bin/bash

echo "Activating swap from swapfile"
sudo swapon /swap/swapfile
sleep 1
echo "Hibernating"
sleep 0.2
xtrlock -b &
sudo s2disk
echo "Returning from Hibernate"
sleep 10
echo "Deactivating swapfile"
sudo swapoff /swap/swapfile
sleep 5
echo "Wiping swapfile"
#sudo wipe -k -f -q -Q 1 /swap/swapfile
sudo dd if=/dev/zero of=/swap/swapfile bs=1M count=24576 status=progress #reserve 24GB
sync
sleep 1
echo "Making swapfs again"
sudo mkswap /swap/swapfile
echo "All done, ready to hiberante again"